The present invention generally relates to a method for controlling a charged particle beam and to a charged particle beam apparatus, and particularly to a method and apparatus for controlling the focus, brightness and/or contrast of the charged particle beam.
In the production/inspection processes for products of function elements such as semiconductor devices and thin-film magnetic heads produced by surface micro fabrication, a scanning electron microscope (hereinafter, referred to as SEM) is widely used to measure the line widths of the formed patterns (hereinafter, simply said as “measure”, “measurement”, “measured” and so on) and visually examine the fabricated surface. The SEM scans the surface of a sample by using an electron beam, detects the secondary electrons emitted from the sample or the electrons reflected from the sample, and uses them to form an image of the scanned region.
The SEM is constructed to create a precise image that is as close to the shape of the sample's surface as possible in order to observe/measure the surface of the sample. The distance between two arbitrary points on the sample's surface can be computed from this created image.
This computation is generally called “critical dimension determination”. The SEM having such computing function is referred to as the critical dimension SEM (CD-SEM). It is known that the electron beam irradiated in the semiconductor-measuring SEM is moderately accelerated, considering the damage to the sample, with energy of some hundreds of electron volts to reach the sample's surface that is to be observed.
Recently, the micro fabrication on the semiconductor surface has been progressed to further miniaturization in which a photoresist is used as a photosensitive material for photolithography (hereinafter, called ArF resist) that reacts to argon fluoride (ArF) excimer laser.
Since the ArF laser light has a short wavelength of 160 nm, it is said that the ArF resist is suited for the exposure of finer circuit patterns. However, from the recent examination, this resist is found very fragile to the irradiation of electron beam because, when the formed pattern is observed or measured by the SEM, the base material of acrylic acid resin induces condensation reaction due to the irradiation scanning of the focused electron beam to reduce the volume (hereinafter, called “shrink”), with the result that the shape of the circuit pattern changes.
On the other hand, the charged particle beam apparatus such as CD-SEM has the drawback that the image is blurred if the beam is not precisely focused on the sample's surface. In addition, if the image does not have sufficient brightness and contrast, it cannot be accurately measured. A document of, for example, JP-A-2001-68048 discloses that the image obtained when an electron beam scans an object being measured is used to make focus evaluation, and that the beam is adjusted on the basis of this evaluation.